1. Field of the Invention
This invention relates to the production of ethanol and, more particularly, to a process for producing ethanol by the fermentation of plant biomass material. Specifically, the present invention relates to a process for producing ethanol by the fermentation of cellulose and hemicellulose hydrolysates using the fungus Paecilomyces sp.
2. Description of the Prior Art is of great interest to a wide variety of industries. In particular, due to past and potential future energy crisis situations, the conversion of such biomass materials to useful fuels such as ethanol is of special interest.
Cellulose and hemicellulose are the two most abundant and renewable raw organic compounds in the world. Together, they compose at least 70 percent of the entire world's plant biomass on a dry weight basis. These raw materials are widely available in the waste from agricultural, forest, vegetable, and food process sources. The efficient recycling of these wastes to useful products, and in particular to fuel such as ethanol, would help to substantially reduce disposal problems as well as to provide an abundant and cheap source of fuel. More specifically, plant biomass generally contains from 40-60 percent cellulose and 30-40 percent hemicellulose, with the balance being lignin. If a process for converting the bulk of the cellulose and hemicellulose to ethanol in high yields could be devised, such a process could provide almost an unlimited supply of fuel.
Generally, cellulose is readily broken down to its glucose and cellobiose hydrolysate by-products by acid hydrolysis or enzymatic hydrolysis treatment. While glucose is readily fermentable by many microorganisms to ethanol, cellobiose has proven difficult, at best, to convert to ethanol. Even then, it is convertible to ethanol only in very low yields. (R. Dekker, Biotechnology Letters, Volume 4, No. 7, Pages 411-416, 1982; R. Maleszka, et al., Biotechnology Letters, Volume 4, No. 2, pp. 133-136, 1982.)
Hemicellulose is likewise readily converted to its various hydrolysate by-products by mild acid hydrolysis or enzymatic hydrolysis treatment. The resultant by-products include various pentoses (xylose and arabinose being the main derivatives), hexoses (mannose and galactose), and sugar acids. By and far, D-xylose is the major hemicellulose hydrolysate constituting approximately 60 percent of the total hydrolysates produced therefrom.
A variety of processes which use different yeasts to ferment xylose to ethanol have been investigated and disclosed in the literature. A prime motivating force behind these investigations is that the fermentation of 5-carbon sugars derived from hemicellulose is extremely important in order to fully utilize biomass material in producing ethanol. Examples of such prior art techniques include U.S. Pat. Nos. 4,511,656, 4,368,268, 4,359,534, and 4,477,569. Unfortunately, these processes do not convert D-xylose to ethanol in sufficient yields and at sufficiently high rates to be efficient and cost effective.
The utilization of plant biomass will require the effective fermentation of all the sugars derived from the hydrolysate by-products of cellulose and hemicellulose. To date, several bacteria yeasts and fungi have been found to be capable of fermenting various pentoses, hexoses, and D-cellobiose derived from lignocellulosic materials. As previously indicated, however, the end-product ethanol concentrations were quite low with respect to typical glucose fermentations. Possible reasons for such low yields of ethanol are that the microorganisms utilized in such prior fermentation processes cannot tolerate high concentrations of xylose, that microorganism activity is possibly inhibited by high ethanol end-product levels, or that the microorganisms used in these prior processes cannot effectively ferment other minor components of hydrolysates including D-galactose, L-arabinose, D-ribose and/or starch. Other possible reasons for low ethanol yields are that the microorganisms used in these processes produce an appreciable quantity of metabolic by-products such as xylitol and arabitol, or that they cannot efficiently ferment both D-cellobiose and D-xylose.
If a microorganism had the ability to ferment both D-cellobiose and D-xylose, it would enable the process of simultaneous saccharification and fermentation of both cellulose and hemicellulose plant biomass to occur. The only process presently known which makes any reasonable attempt at fermenting both of these substances is the process disclosed in U.S. Pat. No. 4,472,501, wherein the microorganisms Kluveromyces cellobioborus or Kloeckera apiculata are used. Unfortunately, only low yields of ethanol are obtained utilizing low concentrations of glucose, xylose, and cellobiose. Thus, if effective and efficient bioconversion of plant biomass is to become a reality, a need still exists for a process which efficiently produces high yields of ethanol from mixtures of cellulosic and hemicellulosic materials at high concentrations.